Evaluation of thyroid function monitoring in people treated with lithium: Advice based on real‐world data

Abstract

INTRODUCTION: Blood test monitoring is essential for management of lithium treatment and NICE guidance recommends 6-monthly serum testing of thyroid function. We examined conformity to these guidelines and impact of monitoring outside these intervals. METHODS: We extracted serum lithium and thyroid hormone results at one centre between Jan 2009-Dec 2020. We identified 266 patients who started lithium during this period with no history of thyroid abnormality within the previous 2?years and were at risk of developing thyroid abnormalities. We examined interval between tests, time between onset of lithium testing and first TSH outside the laboratory reference range and assessed impact of testing outside recommended 6-monthly intervals. RESULTS: The most common testing frequency was 3-monthly (±1 month), accounting for 17.3% of test intervals. Kaplan-Meier analysis showed that most thyroid dysfunction manifests within 3?years (proportion with abnormal TSH at 3?years = 91.4%, 19.9% of total patients). In the first 3 months from commencing lithium therapy, 8 patients developed subclinical hypothyroidism and had clinical follow-up data available. Of these, half spontaneously normalised without clinical intervention. In the remaining patients, thyroxine replacement was only initiated after multiple occasions of subclinical hypothyroidism (median=2?years after initiating lithium, range 6 months-3?years). CONCLUSION: The peak interval at 3 months suggests that thyroid function is frequently checked at the same time as serum lithium, indicating too frequent testing. Our data supports the recommended 6-monthly testing interval and highlights poor adherence to it.